In many applications it is necessary to amplify an AC signal superimposed with a relatively large DC component. In these cases, an AC amplifier fed with the input signal through decoupling capacitors is commonly used. In particular, this is the case with signal channels used for recording data to hard disk drives (HDD).
Two modes of recording data on a HDD are longitudinal recording and vertical recording. Far an AC amplifier, a main difference between these two techniques includes a typical spectrum of an AC signal to be amplified when using the longitudinal recording technique, which differs noticeably from the spectrum of a corresponding signal when using the vertical recording technique.
In comparing the two diagrams of FIG. 1, signals used for recording data on a HDD using the vertical recording technique have a non-negligible power content at very low frequencies. Therefore, the pass band of AC amplifiers optimized for vertical recording should be extended to relatively low frequencies. In other words, they should have a reduced low corner frequency (LCF).
Typically, an AC amplifier is composed of a differential amplifier as shown in FIG. 2. A differential input signal is fed to the inputs INA and INB through respective decoupling input capacitors C. The architecture of this amplifier is very straightforward, but a sufficiently reduced LCF may only be achieved by using relatively large decoupling capacitors C.
Unfortunately, in silicon integrated circuits, when the size of the decoupling capacitors C is increased, the augmented parasitic capacitances CP1 and CP2 between the plates of the integrated capacitors and the silicon substrate on which the amplifier is formed significantly increase the input capacitance of the amplifier.
To keep parasitic capacitances smaller than a maximum acceptable value, these relatively large DC-decoupling capacitors should eventually be fabricated in a metal-dielectric-metal form. However, this means burdening the fabrication process with additional steps and fabrication masks.